(Field of the Invention)
The present invention relates to a turbine rotor having a dissimilar metal weld section, manufacturing method thereof and a steam turbine using the turbine rotor.
(Description of Related Art)
A steam turbine is composed of a boiler, a turbine rotor, moving blades, a generator, and the like.
In a large turbine rotor, high temperature creep rupture strength is required on its high pressure side and tensile strength and toughness are required on its low pressure side in addition to that the axial length becomes long. Therefore, when the steam turbine rotor was to be formed of one material, it was difficult to obtain characteristic satisfying these respective requirements. Particularly, iron-based alloys of prior arts are approaching the limit as a material of the high pressure side rotor accompanying rising of the steam temperature.
Therefore, use of a Ni-based super alloy superior to the iron-based alloys in heat resistance has been studied. However, the Ni-based super alloy is inferior to the iron-based alloy in terms of manufacturability of a large block, workability, cost, availability and the like.
Thus, a manufacturing method is known in which the high pressure side rotor is formed of the Ni-based super alloy material excellent in heat resistance, the low pressure side rotor is formed of the iron-based alloy material excellent in tensile strength and toughness, and these materials are integrated by welding.
However, when a rotor is to be welded, it is necessary to form a weld bead or so-called penetration bead formed on the rear side of the weld section in a butting section which is positioned in the bottom section of the weld section and welded first. When the penetration bead is not formed, a non-welded section remains in the butting section which may amplify the rupture potential during operation.
Therefore, when the rotor is to be welded, it is necessary to form the penetration bead. However, when combination of the rotor material is of dissimilar materials particularly combination of the Ni-based alloy material and the iron-based alloy material, difference in thermal property value of the both becomes a problem. In this case, (1) movement of an element due to difference in the additive element, (2) securing the mechanical property even when the heat treatment condition is different and (3) formation of the penetration bead even when the thermal conductivity is different become the problems.
With respect to the problems of (1) and (2), solutions are described in Japanese Unexamined Patent Application Publication No. 2008-215181 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2000-254774 (Patent Document 2), Japanese Unexamined Patent Application Publication No. 2000-254776 (Patent Document 3), and Japanese Unexamined Patent Application Publication No. 2009-248095 (Patent Document 4).
Patent Document 1 discloses a turbine rotor using 12% Cr-based steel having a totally tempered martensitic structure and Cr—Mo—V-based steel having a bainitic structure as rotor base materials with the butting section thereof being joined to each other via a weld section, in which the hardness distribution of a buttering layer arranged in the butting section is kept within a predetermined range.
Patent Document 2 discloses a technology of a method for welding dissimilar materials of welding and joining Cr—Mo—Fe-based heat-resistant steel having been used for a long period in a hydrogen gas atmosphere with a new Ni—Cr—Fe-based heat-resistant alloy in which the vicinity of the joining section of the Cr—Mo—Fe-based heat-resistant steel is subjected to a dehydrogenation treatment by being heated to a predetermined temperature.
Patent Document 3 discloses a method for preventing a stress corrosion crack of a weld section of piping inside a nuclear reactor forming a fusion/solidification layer within a range of a predetermined depth from the inner surface or the outer surface of the weld section when stainless steel pipes are to be butt-welded.
Patent Document 4 discloses a method for welding two kinds of Ni-based welding materials one by one with respect to steel.
Japanese Unexamined Patent Application Publication No. 2012-61498 (Patent Document 5) discloses a technology for setting a rotor shaft of low alloy steel and a turbine vane of a Ni-based super alloy integrated by fitting a projection and a concavity to each other in an aligned state, and thereafter subjecting the butting portion to welding over the entire periphery while being rotated around the axis of the rotor shaft.